Datasheet



SLUS610 − JULY 2004
23
www.ti.com
APPLICATION INFORMATION
Using the PG Output
The PG
output is an indication of the load power status. PG is asserted after a load turn-on, once the load voltage
has ramped up to the input dc level, as indicated by a small V
DS
drop across the pass FET. The load voltage
is sensed by the DRAINSNS pin, which is connected to the pass FET drain through a small-signal blocking
diode. Also, the TPS2393A first confirms that the full programmed sourcing current (typically 40 mV/R
SENSE
)
is available to the load electronics prior to declaring power good. The PG
status is latched once the power
conditions are met, so that momentary current limiting operation due to input supply transients is not reflected
in this output status. This pin can be used to enable downstream converters, provide a visual indication of load
power good, or be level-translated or optocoupled to provide status reporting back to the host controller.
When using PG
to drive the enable input of a converter, care should be taken not to exceed the manufacturer’s
maximum voltage ratings for the pin. When asserted, the output driver pulls the PG
pin to the −VIN pin potential.
Because this status in latched, subsequent current limit operation of the circuit could result in pulling the enable
input below the brick’s VIN− potential during the fault timeout period. If the brick does not provide an internal
clamp on this pin, a diode can be connected as shown in Figure 28 to externally limit the swing below VIN−. In
either case, a resistor (R7 in Figure 28) should be used to limit the current pulled from this pin, protecting both
the converter and the PG
output. R7 should be large enough to limit the PG input current to less than 10 mA,
while still allowing the brick enable to be pulled below its maximum V
IL
threshold.
10 µA
UDG−20177
GATE
12
13
10
9
ISENS
DRNSNS
RSENSE
VOUT+
VOUT−
VIN+
VIN−
DC/DC
CONVERTER
Q1
8
−VIN
D1
BAS19
11
RTN
D3
V
DD
R7
43 kΩ
TPS2393A*
C
IN
EN
−48
V
GND
PG
*Additional details omitted for clarity.
Figure 26. TPS2393A Active-Low Converter Enable